Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide in the presence of heterogeneous or homogeneous catalysts into equimolar amounts of phenol and acetone, a co-product. However, the world demand for phenol is growing more rapidly than that for the acetone co-product. In addition, due to developing shortages in supply, the cost of propylene is likely to increase.
Alternatively, phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. This reference patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
Although the production of phenol and cyclohexanone from cyclohexylbenzene appears to be analogous to the Hock process for producing phenol and acetone from cumene, the chemistries in each step are actually very different. For example, the chemistry of the cleavage of cyclohexylbenzene hydroperoxide is much more complicated than that for cumene hydroperoxide and more by-products (both in types and amounts) can form. Thus, cleavage of cyclohexylbenzene hydroperoxide to phenol and cyclohexanone is acid catalyzed and, although a variety of acid catalysts can be used, sulfuric acid is preferred for its low cost and easy availability. However, significant yield loss to by-product (both primary and secondary) can occur in the sulfuric acid-based cleavage of cyclohexylbenzene hydroperoxide if the acid strength in the reaction medium is not appropriate. Primary by-products may include the β-scission products such as hexanophenone and 6-hydroxylhexanophenone (6-HHP). Examples of secondary by-products include those derived from cyclohexanone, such as 2-(1-cyclohexenyl)cylohexanone and 2-(cyclohexylidene)cyclohexanone (cyclohexanone aldol condensation products), 2-hydroxycyclohexanone and cyclohexenone (cyclohexanone oxidation products). Formation of the primary by-products result in loss of both phenol and cyclohexanone; while secondary by-products further reduce yield to cyclohexanone.
The ability to monitor and adjust acid strength in the cleavage medium is desired. However, there is no method to measure the acid strength in the cleavage medium. Conventional methods to measure acid strength in solution include pH measurement (for aqueous solutions) and ion conductivity measurement. However, these methods are not sensitive enough for the low acid concentrations (e.g., up to thousands of ppm) used in cleavage process and are not sensitive enough to determine the difference among a wide range of solution compositions.
Therefore, a need exists for a detection method that addresses one or more of the above identified issues.